The ABR (available bit rate) service in ATM (asynchronous transfer mode) networks is intended to make the best use of remaining capacity after higher priority services such as CBR (constant bit rate) and VBR (variable bit rate) have been provided for. ABR employs a closed-loop flow control mechanism based on RM (resource management) cells to allocate and moderate user access to the available bandwidth. The flow control loop can be end-to-end in which case the RM cells travel all the way from source to destination before being looped back, or it can be segmented into smaller control loops with interim switches emulating the behavior of the end systems. Such interim switches are referred to as VS/VD (virtual source/virtual destination) switches. The RM cells provide information regarding the congestion level in the switches in the path and regarding the bandwidth allocated to individual sources. This information is used by the source to modify its transmission rate, the objective being to utilize link capacity fully while not losing any cells as a result of congestion. ABR is not intended for real-time applications, and no guarantees are made with respect to cell delay and cell delay variation.
Each RM cell contains a CI (congestion indication) parameter and an NI (no increase) parameter which may be adjusted as the RM cells pass through the switches in the path in either the forward or backward direction. The CI/NI parameters contained in the RM cell when it returns to the source are used to indicate to the source what type of relative increase or decrease should be effected to the rate at which the source can send cells. More specifically, the source is allowed to send cells at a rate entitled the ACR (allowed cell rate), and it is this ACR which is iteratively adjusted by the source each time it receives a returned RM cell as a function of the CI/NI parameters.
There have been many proposed methods for calculating CI/NI in an ATM switch. See for example U.S. Pat. No. 5,633,859 which issued May 27, 1997 to Jain et al. entitled "Method and Apparatus for Congestion Management in Computer Networks Using Explicit Rate Indication", PCT application PCT/US/19720 published Jun. 5, 1997 to Jones, entitled "Controlled Available Bit Rate Service in an ATM Switch", and PCT application PCT/US96/08906 published Dec. 12, 1996 to Jones entitled "Controlling the Flow of ATM Cells in an ATM Network".
It is not immediately clear that any of the algorithms in the above identified references feature a desired combination of characteristics including fast convergence, fairness of elastic bandwidth allocation between virtual connections (elastic bandwidth being the amount of ABR bandwidth available after accounting for the MCR for all connections), low hardware cost and complexity as well as short processing time, scalability with number of virtual circuits, robust against dishonest sources who lie about their current cell rates, high link utilization, capable of taking other traffic classes into consideration, being easily extendable to support VS/VD (virtual source/virtual destination), and capable of redistributing unused elastic bandwidth as well as unused MCR.